1. Field of the Invention
The present invention relates generally to a system and method for extracting bitumen from tar sands and more particularly to an improved system and method of bitumen extraction that enable the operation of a continuous, cost-effective and reliable process for such extraction.
2. Description of the Related Art
A variety of prior art patents teach or disclose various processes for extracting bitumen from tar sand flows, and thus producing crude for further refining and processing, as is known in the art. For example, U.S. Pat. Nos. 3,941,679 to Smith et al., 4,120,775 to Murray et al., 3,856,474 to Pittman et al., 1,862,945 to Schlotterhose, and 1,024,230 to Turner et al. each teach systems and concomitant apparatus for separating hydrocarbons from tar sands and the like.
Furthermore, U.S. Pat. No. 4,311,561 to Hastings, incorporated herein by reference, offers further improvements to the prior art. The '561 reference teaches an improved system for extraction of bitumen from tar sands by flowing the tar sands and a suitable solvent in a first direction through a plurality of serial extraction chambers while a flow of extracted bitumen and solvent are moved counter to the flow of the tar sand feedstock. The feed stock of tar sands are fed into an initial extraction chamber utilizing a hopper, conveyor, or similar feeding apparatus, as is well known in the art. Solvent is introduced in the “last” extractor in the series, whereby the flow of solvent—and thus extracted bitumen—is counter to the flow of the tar sands through the extraction chambers.
In this fashion the amount of bitumen contained in the tar sand in each successive extraction chamber is reduced until, in the final extraction chamber, only sand and solvent remain. At this point in the process the sand and solvent are treated with hot water to remove (or separate out) the solvent from the sand, which solvent may then be reused in the process.
Each extraction chamber comprises a an agitator for effecting mixing of tar sand and solvent and further with a circulating system for bringing solvent and bitumen solution from the top of the chamber for discharge into the bottom thereof adjacent the agitator. The tar sand and solvent mixture in each extraction chamber is removed therefrom and discharged into a successive chamber by operation of a conveyor extending from the bottom of each chamber to the top of the next successive chamber.
In this fashion, the extracted bitumen and solvent are removed from the “first” extraction chamber in the chain of extraction chambers for further processing and, ultimately, the refining of hydrocarbon products from the bitumen.
One difficulty with prior art systems such as those described and discussed briefly herein above is the continued presence of particulates or fines in the extracted bitumen and solvent mixture. The presence of these particulates is highly undesirable since they make the continued processing of bitumen considerably more difficult, requiring various and sundry apparatus for their removal. Accordingly, it is readily seen that an extracted bitumen and solvent solution or mixture having a very low particulate content is highly desirable for clean and efficient production of hydrocarbons.
Additionally, one further difficulty with the aforementioned process is the situation where the moisture content of the tar sands entering the extraction process is variable, either too high or too low, due to the presence of environmental moisture. For example, when the tar sands are subjected to a down pour of rain, or snow-covered prior to entering the process, the moisture content thereof will be considerably higher than when the feed stock is fed into the system on a dry day. This wide variability in moisture content causes great difficulty in keeping a steady-state flow of extracted bitumen, which leads to many process delays and even shut-downs. These inefficiencies are of course quite costly, both in lost production and labor required to restart or re-balance the process.
One additional disadvantage with the prior art processes discussed above is the high cost of the solvent necessary to extract the bitumen from the tar sands and the relative inefficiency of these systems at recovering the solvent from the bitumen-depleted sand, thereby greatly enhancing cost. Accordingly, there is a great need in the art for a bitumen extraction system and method that provides for efficient solvent recovery throughout the process.